Your site is considered a UK site, and you are getting a boost in rankings in the UK as a result.

You need to ask yourself what country you really want to target. If the answer is both, often the best approach is to either create a different site, or to create a different section of your website targeted at each specific country.

If you are more interested in targeting the US, then you need to change the geotargeting to the US. That fastest way to do this (assuming you don't have a .uk) is to use Googles webmaster tools. Other options include moving servers (basically, getting a US IP).

The one final option is to simply work really, really hard on your SEO, so that even though you are not getting a boost in the US, you show up well anyway. Mostly, this would be link building, and US links would not hurt, but I don't think they would help much, since they go against the UK geotargeting you have.